The present invention relates to oscillators, and more particularly to the reduction of step transition times for a YIG oscillator having an FM filter capacitor for noise reduction by predistorting a drive current for the YIG oscillator.
In high frequency applications requiring frequencies in the gigahertz range YIG (yttrium-iron-garnet) oscillators are used. These oscillators consist of YIG spheres that are subjected to high magnetic fields, the strength of the magnetic field determining the frequency of operation of the YIG spheres. A drive circuit for a YIG oscillator has a main tuning coil having distributed inductance L and resistance R that is driven by a current source. The strength of the magnetic field is a function of the amplitude of the current i(t) from the current source. A capacitor C is sometimes placed in parallel with the main tuning coil to stabilize the YIG oscillator by filtering noise present on the main coil tuning current i.sub.L (t). An equivalent circuit for a YIG oscillator having a stabilizing capacitor is illustrated in FIG. 1. The capacitor C has the disadvantage of significantly extending the time required for the oscillator to restabilize after a step frequency change, i.e., a step change in the drive current i(t). As shown in FIG. 2 the drive current i(t) from the current source is modified by a transfer function that is a function of the circuit components to produce the main coil tuning current i.sub.L that has appreciable ringing at the edge of the step transition so that the frequency is not stable until some time, dt, after the step change is initiated. In other words EQU i.sub.L (t)=T*i(t)
where T is the transform function T=f(R,L,C).
What is desired is a method and apparatus for reducing the step transition times for a YIG oscillator with a filter capacitor.